1. Field of the Invention
The present invention is generally related to nuclear fuel assemblies and, in particular to an apparatus and method for loading fuel rods into interior grid cells of the fuel assembly.
2. General Background
Grids used in commercial nuclear fuel assemblies are fabricated with stops that grip the fuel rods in place when the fuel assembly is in the as-built condition. The stops on two adjacent cell walls are hard stops, with a fixed location, and the other two cell walls have spring loaded soft stops. During fuel assembly fabrication it is desirable to have the soft stops withdrawn away from the centerline of each grid cell to allow free passage of the fuel rod into and through the grid. Keying of the stops to cause them to withdraw from the centerline of their grids is typically performed using a rectangular wire that is inserted into and through the windows of the grid. The wire key extends through the full width of the grid and is rotated so that the thickest cross section of the key wedges the strip section backward and thus withdraws the stop. The key is machined with cutouts to preclude interference with the fuel rods when in the rotated position. Several interior grid cells, however, can not be keyed using the rectangular keys. This is due to interference with previously installed guide thimbles. Guide thimbles that have ferrules attached to restrain the grid must be installed prior to insertion of the fuel rods. The guide thimbles have a larger diameter than the fuel rods and the rectangular keys do not have clearance to get past the thimbles. As a result, several fuel rods must be installed into unkeyed grid cells. A lubricant is used to minimize damage to the fuel rods, but the lubricant is difficult to remove from the fuel rods after assembly.
Patents directed to keying of grid cells that applicants are aware of include the following.
U.S. Pat. No. 3,933,583 discloses rectangular wire keys used for deflecting stops in the grid.
U.S. Pat. No. 4,651,403 discloses a pair of comb devices that are utilized to depress springs within the grid cells for loading of fuel rods.
U.S. Pat. No. 5,068,081 discloses the consecutive use of two separate members in the grid wherein the first member is removed after the second member is inserted and the fuel rods are then loaded into the grid.
U.S. Pat. No. 5,124,116 discloses a grid key for keying exterior grid cells only.
The known art does not address the need for a grid key that may be used on interior grid cells when the soft stops can not be retracted from the adjacent cell due to interference from associated equipment such as a guide thimble.